Yup, a combination of things, depending on just how cold it is. A balaclava with a scarf or neck gaiter does a pretty good job most of the time. When temps get down into the single digits and a bit below zero (yes, I bike in those temps), I also find a neoprene nose/mouth mask to be very helpful, because that's a sensitive part of the face for me. The other part of my face that is very sensitive in extreme cold are my eyes. Then I add a good set of tinted ski goggles. The tinting really helps wear and tear on the eyes on sunny days with the light reflecting off all the snow.

Can't help but think how I must look like a fright with such a setup, but all our neighbors know it's me under all that stuff and wave when they see me. Haven't been racking up a lot of miles, but I am proud to say that I'm biking every day, regardless of the weather, doing 5 to 10 miles a day, now that I have the studded tires. Biking all bundled up, plus the cold, makes for more pedaling effort, but I love it so. It's its own kind of biking. Nothing else like it.